As stated in the Known Issues list, Internet Facing Deployment (IFD) is required if you are using Microsoft Dynamics CRM 2013 (on-premises version) with your CRM for Tablets app.
Your system administrator must configure claims-based authentication before your users can access their Microsoft Dynamics CRM data with their CRM for tablets app. If you have your Microsoft Dynamics CRM website available over the internet but it is not using the Microsoft Dynamics CRM IFD configuration, it is not supported. To verify that your on-premises deployment is configured for IFD, open Microsoft Dynamics CRM Deployment Manager on your Microsoft Dynamics CRM Server. The Authentication Summary section should show that both Claims-Based Authentication and Internet-Facing Deployment are enabled.
To ensure you have a smooth login experience from the outside of you network please make sure you have disabled Integrated Windows Authentication (IWA) on you ADFS server so that Forms Authentication is happening.
- Known Issues for CRM for Tablets and Phones - link